2.13 Power Tips: Higher voltage LEDs improve efficiency

Hi. I'm Robert Kollman. I'm a senior applications manager at Texas Instruments. Welcome to Power Tips.
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Hi. It's Robert Kollman again. This Power Tip 36 is going to talk about driving high voltage LEDs offline. In this slide, we see some products that are currently on the market. These are LED replacements for light bulbs. On this surface of the LED light, you can see the individual LEDs, and then on the back side of the light, you can see where a power supply is put into the packaging.
Another interesting aspect of this is that you can see that there's significant heat sinking. And this LED bulb only consumes about 10 watts of power, but cooling the LEDs is very important for their life. These LEDs might have a lifetime of 50,000 hours at 80 degrees C, and they might go to 30,000 hours at 90 degrees C. So it's very important to get as much heat out of the LED bulb as possible.
Up until recently, we had been doing buck regulators to drive the LED bulbs. These buck regulators would take 120-volt input. We would filter it for MI, and then we would put it through an MI filter, and then through a set of rectifiers on the output of the MI filter. Do a slight bit of filtering on the rectified DC, and then provide that to our power stage. Again, our power stage is an upside down buck. You can see the power switch here. That's Q2. And you can see the diode, the freewheeling diode in the buck regulator. That's D3. Here's the output inductor.
And then here's our output voltage, and one interesting thing you'll note in this design is that there are no electrolytic capacitors. And this is very important, because that little power supply we saw in the previous slide gets tucked away inside a sealed container, and it has no cooling. And so the temperatures in the LED light bulb can get quite high. In fact, we've measured 125-degree power supplies in some bulbs that we took apart.
And so 125 degrees C and long life that you'd like to get out of LED light bulbs are not supporting each other. Most capacitors only have a rating of 105 degrees, so it's very critical that you keep this thing as efficient as possible, and it's critical to keep the electrolytics out of it also.
Some of the other things that you can see in this circuit-- again, our buck regulator, our control IC that provides the PWM signals to the buck regulator. This little bit of circuitry here is linear power supply, and it just converts the DC input voltage down to a voltage usable by our control IC. This circuit actually generates a constant current through this resistor here, and then that current shows up to power our control circuitry.
Another little thing in this circuit is you can see that this transistor here, and you see that we have a 1k connected across the input line when it is on. And the reason we do this is to provide some loading on the output of the triac dimmer. The triac dimmer has a triac that is used as a switch, but it also has a capacitor across that triac that serves as an EMI filter. It also provides a current path for some of the electricity to flow from the input through the power supply, and without this resistor, it is very hard to detect when that triac has fired and when it has not. And you get pretty sloppy dimming if you don't have this resistor and transistor in here.
This circuit here actually helps the light shape the current waveform. So with this at full duty effect, we end up with a pretty good power factor with this circuitry. Now recently, there have been some LED manufacturers that have gotten away from low voltage LEDs. Typically, the white LEDs we've used in the past had a forward voltage drop of about three, and we would string five to nine of them up and have a 15 or 20 volt power supply, which is real nice for buck regulator offline.
The problem with that, that circuit was running at a fairly high current. It had to handle the input voltage, and then it also had to handle the output current of the power supply. So if we had a 700-milliamp output, everything, the power transistor and the diode and the inductor, would be running at that output current. So it's a combination of high voltage and high current on the switches.
So it impacted the efficiency and the cost of the power supply. Now recently, some LED manufacturers have realized that maybe you can end up with a better power system if you go to a higher voltage LED. And they have, and there are products available that are 50 volts. And if you series connect several of those, you can get to a requirement for 300 volt output power supply. And that's what we have here.
And in this case, the output voltage is always higher than the input voltage. And so you can use a boost regulator. And now the boost, the currents are down, because in this particular case, we're only supplying 34 milliamps output current. And so that's really the reason you can improve the efficiency with this circuit.
You can see that this is pretty much similar to the previous circuit. We have the linear regulator here. We have a circuit that we're doing a little bit of wave shaping of the current waveform in the power supply to improve the power factor. But the thing we don't have is we don't have that dummy load resistor that we had in the previous slide. And so that gives us some cost reductions and gives us an efficiency improvement also.
Here are two circuit boards comparing the area required by the buck and the boost topologies. In the top one, here's the buck regulator, and in the bottom one, here's the boost. And you can see the impact that we were talking about from going from buck to boost. This resistor here is the dummy load resistor that we had to have in the buck that we didn't need in the boost. You can see that the inductor size has significantly been shrunk. That's because it's reduced stresses on this inductor compared the other one. And you can see fewer components and lower cost, lower size.
So to kind of summarize, the buck has more complements and is larger than the boost. And the reason for most of that is the dummy load required by the buck regulator. Also adding to that is the fact that we have reduced requirements on the inductor too. So we end up with about a 5% efficiency improvement in the power supply with the boost, and this means one half less loss in the power supply, and so that reduces the total dissipation of the bulb and leads to longer life.
Additionally, we found that the boost has better power factor, and when we use it with a dimmer, the dimming is much smoother than the buck. For more power tips, visit Power Management DesignLine, and search for power tips, or click on the link to all articles in the description section is video. Thank you.